1. Technical Field
The invention relates to cable television (CATV) interdiction systems and, more particularly, to a method and apparatus for controlling the jamming parameters in such systems.
2. Description of the Prior Art
In the past, a scrambler has been provided to encode premium television channels at a headend of a cable television system. The applied scrambling precluded reception by an unauthorized converter/decoder at a connected premises. Data representing the channels or tiers of programming to which the subscriber was entitled were addressably transmitted to a particular converter/decoder and stored in an authorization memory. As a result of the addressed transmission, a subsequently transmitted program would be authorized by selectively enabling the decoder portion of the converter/decoder to decode the scrambled premium channel or program.
The provision of one scrambler per premium channel at the headend and the inclusion of a descrambler in each converter/decoder at the premises of the television receiver was particularly expensive. Furthermore, providing a converter/decoder on premises has turned out to be a great temptation to service pirates who imaginatively seek ways to receive preminum channels. As a result, cable television equipment manufacturers have entered into a veritable war with such pirates resulting in complicated service authorization protocols, which in some instances involve multiple layers of encryption by both in-band and out-of-band data transmissions thereby further increasing the costs of the converter/decoder. In addition, scrambling systems may leave artifacts in the final signal.
Consequently, the cable industry has reviewed other technology developed in the early stages of cable television, such as the application of negative and positive traps, and more recent techniques, such as interdiction, to improve CATV systems.
A relatively recent technique for premium channel control is the interdiction system, so called because of the introduction of an interfering signal into a premium channel at the subscriber's location. Most embodiments consist of a pole-mounted enclosure located outside the subscriber's premises designed to serve one or more subscribers. This enclosure contains at least one microprocessor controlled oscillator and switch control electronics to secure several television channels. Control is accomplished by injecting an interfering or jamming signal into unauthorized channels from this pole-mounted enclosure.
For the sake of efficiency, it is known to utilize one oscillator to jam several premium television channels. This technique not only reduces the amount of hardware required, but also maximizes the system flexibility. The jamming signal frequency is moved as a function of time from channel to channel. The oscillator is frequency agile and hops from jamming one premium channel frequency to the next. Cable television channels and, of course, premium service channels may extend over a wide range of frequencies, for example, from 54 Mhz. up to and including 1000 Mhz. Thus, if only one oscillator were provided, it would have to be frequency agile over a wide range.
One such system, illustrated in U.S. Pat. No. 4,450,481 by Dickenson, has a single frequency agile oscillator which provides a hopping gain-controlled jamming signal output to four high frequency electronic switches. In this system, each switch is associated with one subscriber drop. Under microprocessor control and depending on which subscribers are authorized to receive transmitted premium programming, the microprocessor selectively gates the jamming signal output of the single oscillator via the switches into the path of the incoming broadband television signal to each subscriber. Consequently, an unauthorized subscriber upon tuning to a premium channel will receive the premium channel on which a jamming signal of approximately the same frequency has been superimposed.
A significantly more advantageous interdiction system is disclosed in U.S. Pat. No. 4,912,760 by West, Jr., et al. which controls a plurality of frequency agile oscillators for each subscriber. Each voltage controlled oscillator is allocated a continuous band of frequencies consistent with the elimination of jamming signal harmonics which could disturb authorized programming at a higher channel frequency. The interdiction apparatus includes generating and storing frequency control words for operating the voltage controlled oscillators consistent with a headend selected jamming factor for a particular channel to be jammed and addressably transmitted and stored premium programming authorization data. In U.S. application Ser. No. 07/476,041, filed Feb. 6, 1990, by West, Jr., et al., the method of programming the time slots of the West I system to vary the dwell time and jamming factor of the jamming signals for different premium channels is disclosed.
The system in West I and West II is configured in such a way that different channels can be programmed with a variable jamming factor. This provides a channel with a programmable number of time slots which can be assigned and a variable dwell time for a given hopping rate. This variability is necessary because it has been recognized that the effectiveness of a jamming signal should be different for specific channels because not all channels have the same information content. Interdiction works most effectively by destroying the entertainment value of the channel and can be accomplished in a number of ways.
For example, channels which have considerable video content may have to be jammed with a larger number of time slots than those which do not. This is true for many types of premium viewing including adult type channels where the video portion should be entirely obscured. However, for many other types of channels jamming with single or a small number of time slots provides enough interference with the channel information to discourage watching unless the jamming signal is removed.
It has additionally been found that other types of variations in the jamming parameters of an interdiction system can increase jamming effectiveness. A pulse jamming method has proved to be effective in many instances where total obscuration of the video is not needed. In pulse jamming, the jamming signal for a specific channel is turned on and interdicts the channel for a particular duration and then is turned off for a particular duration. This pulsing of the jamming signal provides enough interference with the channel that, even when the channel is in the clear for short periods of time, its entertainment value is lost. Pulse jamming is more difficult to do in a time slot based system such as West I and West II because the jamming energy is already being time shared between several channels. To have an effective and efficient interdiction system, the jamming energy available when a channel is in the clear when operating in pulsed mode should be reallocated to other jammed channels without having to reprogram the time slots. Therefore, it would be advantageous to provide a method of combining pulse jamming with the programmable time slot jamming technique of West I and West II.
Further, it has been noted that the hopping rate can affect the jamming effectiveness of a system. For channels with a greater jamming factor (more time slots) it is more effective to have a faster hopping rate. For channels with a lower jamming factor (less time slots) it is more effective to have a slower hopping rate. With faster hopping rates and more time slots the obscuration of the video signal is ensured. With a slower hopping rate and less time slots, many television receivers tend to lose vertical synchronization so that the video portion of the program will roll.
Because of the flexibility in the channel line-up and multitude of service tiers demanded by today's subscribers, system operators need to have a combination of these methods to discourage unauthorized viewing of premium channels. It would be advantageous to provide an interdiction system with a variety of variable jamming parameters such as variable dwell time, variable hopping rate, and pulsed mode jamming so as to be able to modify jamming effectiveness based on channel line-ups, number of pay channels and other system factors. It would be further advantageous to provide combinations of these variable jamming parameters to generate the optimal jamming effectiveness for a particular system configuration.